The regulations against the emission of NOx caused by combustion are intensified year after year, and very active scientific experimentation has been conducted to decrease NOx emissions. The NOx generated by combustion includes fuel NOx, prompt NOx and thermal NOx. Among these types of NOx, thermal NOx is produced as the nitrogen molecules in combustion air are oxidized in a high temperature atmosphere, and is highly dependent on temperature. At higher combustion temperatures, NOx production increases sharply. Thermal NOx is produced without fail if the combustion gas contains nitrogen molecules, and especially when a hydrocarbon-based fuel is burned, the NOx emitted is mostly thermal NOx. A number of methods for decreasing NOx have been proposed, including multi-stage combustion methods, exhaust gas recirculation methods, lean combustion methods, etc., and it has also been proposed to combine these methods in many ways.
In multi-stage combustion methods, the fuel or combustion air is divided for combustion in two or more stages, which is intended to achieve low NOx combustion by keeping the flame temperature low, or by keeping the oxygen concentration low. The problem here is that the use of these multi-stage combustion methods necessitates the use of a complicated burner.
The exhaust gas recirculation methods are intended to lower the flame temperature or lower the oxygen concentration by mixing part of the combustion gas with the combustion air or fuel, and includes forced exhaust gas recirculation methods and self-induced exhaust gas recirculation methods.
The forced exhaust gas recirculation methods which use a recirculation duct and blower to forcibly mix part of the combustion gas with the combustion air or fuel are the most general methods used. In the self-induced exhaust gas recirculation methods, a specially devised burner is used to let combustion air flow or fuel flow encapture the combustion gas for mixing to achieve the effect of exhaust gas recirculation by the jet entrainment or encapturement. The self-induced exhaust gas recirculation methods have an advantage in that the effect of exhaust gas recirculation can be obtained without forcibly recirculating the combustion gas, and is free from the complication of the multi-stage combustion methods that the fuel or combustion air is divided into a plurality of lines.
A burner using a self-induced exhaust gas recirculation method is disclosed, for example, in Japanese Patent Laid-Open No. 87-17506, and many other burners use the self-induced exhaust gas recirculation methods. However, these methods are limited in the capability to decrease NOx, and further technical development is necessary to meet the latest severe NOx regulations.
Combustion methods developed to maximize the advantage of self-induced exhaust gas recirculation are proposed in Japanese Patent Laid-Open No. 89-300103 and 91-91601, and Japanese Utility Model Laid-Open No. 77-61545. These combustion methods are characterized in that combustion air flow and fuel flow are separately and independently injected into a furnace with a burner without any flame stabilizing mechanism, to maximize the effect of self-induced exhaust gas recirculation.
In this configuration, the flame is not stabilized in the burner, but is formed at a lifted position, and combustion begins after part of the combustion gas in the furnace has been sufficiently entrained or encaptured by the fuel flow or combustion air flow. In these combustion methods, the flame is a gentle diffusion flame, but since there is no flame stabilizing mechanism, stable ignition cannot be achieved without a high temperature. Therefore, even though the methods are suitable for high temperature furnaces such as heating furnaces and melting furnaces, they have problems in that the amount of unburned combustibles increases and a larger furnace must be used for perfect combustion, when they are applied to boilers and low temperature heating furnaces.
Another method for decreasing thermal NOx is to use a premixed flame. Premixed combustion at a high excess air ratio can significantly decrease NOx, but since excessive air increases in the combustion at a high excess air ratio, the efficiencies of combustion and heat transfer greatly decline. Furthermore, the premixed flame is poor in stability.
A method of decreasing thermal Nox by combining the premixed combustion with the effect of self-induced exhaust gas recirculation has been proposed in Japanese Patent Laid-Open No. 91-175211. In this combustion method, a flame specially devised stabilizer is used, and part of the low temperature combustion gas is mixed with a premixture before the premixture initiates combustion, to lower the flame temperature, or to lower the oxygen concentration, for decreasing NOx. This combustion method and apparatus also has problems observed with other premixed type burners, such that an air-fuel mixer is necessary to generate a premixture for premixed combustion. Since a premixture within inflammable limits is used, the flame may go back into the burner or mixer.
Furthermore, there is a problem that since part of the combustion gas is mixed with an inflammable premixture, ignition occurs immediately after the mixing between the premixture and the combustion gas if the mixed combustion gas is high in temperature, barring the effect of self-induced exhaust gas recirculation. Therefore, the flame stabilizer must be specially devised to ensure that the premixture is not ignited when the premixture and part of the combustion gas are mixed.
As described above, self-induced exhaust gas recirculation methods have advantages in that the burner can be simple and low NOx combustion is possible, compared with other low NOx combustion methods such as multi-stage combustion methods and lean premixed combustion methods. In the combustion methods for decreasing thermal NOx by using self-induced exhaust gas recirculation, if the self-induced exhaust gas recirculation is used to the maximum extent for the diffusion flame, the operative temperature range in the furnace is limited, and the usable combustion equipment is also very limited. Moreover, the application of self-induced exhaust gas recirculation to the premixed flame has the problem of flame stability peculiar to the premixed combustion like the back combustion, and disadvantageously requires a more specifically devised flame stabilizer.